Photoresists are materials which change their solubility in a developer solution after the photoresist has been exposed to actinic radiation, such as to ultraviolet, electron or ion beam, laser or X-ray radiation. Photoresist compositions comprise a photoactive compound, a film forming polymeric resin and a solvent. The photoresist composition is applied to a substrate which is to be patterned and the solvent is then removed, as with heat, leaving the photoresist is a thin film covering the substrate. As a consequence of the exposure to radiation of the photoresist, a different solubility rate results between the exposed and unexposed (masked over) portions of the photoresist film which yields a surface relief pattern after development. Those photoresists which become more soluble in a developer solution in the exposed regions are referred to as "positive" photoresists. Those which become less soluble in the exposed regions are referred to as "negative" photoresists. The present invention deals with a class of those photoactive compounds suitable for use in photoresist compositions.
Positive photoresist films may comprise an aqueous alkali soluble resin, such as a novolak resin or a poly(hydroxystyrene), and a photoactive compound. It is known to the skilled artisan to produce positive photoresist compositions such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470. These include water insoluble, aqueous alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resin and photoactive compound are applied, such as by spin coating, spray coating, or other suitable means, from an organic solvent or solvent mixture onto a substrate, such as a silicon wafer or a chrome-plated glass plate. The developer used to process the positive photoresists are aqueous alkaline solutions, such as sodium metasilicate, potassium hydroxide, tetramethyl ammonium hydroxide and ammonium hydroxide. The developer removes the areas of the coated photoresist film that have been exposed to light or other form of irradiation so as to produce a relief pattern in the photoresist film.
The application of a photosensitive film to various substrates is an essential step in the fabrication of integrated circuits. The substrates are generally silicon wafers which may have a thin oxide coating or other coating such as silicon nitride or aluminum. The photosensitive film is used to pattern the substrate in a series of steps including exposure (through a mask pattern), development to yield a relief pattern in the photoresist layer and a substrate etch step to transfer that pattern into the substrate material. It is essential that the mask pattern be accurately reproduced in the substrate etch pattern. To achieve this high degree of accuracy, the mask pattern must be well resolved by the photoresist layer. The characteristics of the photoresist compositions, which are important in commercial practice, include its photospeed, contrast, resolution (edge acuity), thermal stability of the image during processing, processing latitude, line width control, clean development and unexposed film loss.
Not only is the performance of the photoresist critical, but also its quality, specifically, purity and consistency. Both soluble and insoluble impurities lead to yield losses during the manufacture of integrated circuits. Some of these impurities originate from the synthesis of the photoactive compound. The synthesis of the photoactive compound, typically those that are formed by an esterification reaction between a hydroxy containing compound and a halide containing compound, require a basic component to drive the esterification reaction. It is well known in the art to use soluble organic amines, such as 1,4 diazabicyclo 2,2,2! octane, N-methyl morpholine, triethyl amine or diethanolamine to drive the reaction between phenolics and diazonaphthoquinone sulfonyl chlorides, in the presence of a solvent. In order to obtain high degrees of esterification, which are desired for high resolution photoresists, greater than equimolar quantities of the amine are used. These amines form salts, and most often, chloride salts, that are difficult to remove and can lead to insoluble particles in the photoresist film. This patent discloses a method of making photoactive compounds that requires little or no soluble organic amines and furthermore can provide photoresist films with low levels of impurities, especially chloride.